As structural materials capable of being used under a high-temperature and high-pressure environment such as 600° C. or higher, an Ni-based alloy and an Fe—Ni-based alloy having excellent high-temperature strength may be mentioned. An Ni-based alloy has an excellent high-temperature tensile strength and creep characteristics and the alloy capable of being used even at a high temperature of 700° C. or higher has been developed and used in power generation plants and jet engine components. However, since an Ni-based alloy tends to cause macroscopic segregation during ingot production, it is considered difficult to produce a large ingot free from segregation. As heat-resistant alloys from which it is relatively easy to produce a large ingot, for example, Inconel (trademark, the same shall apply hereinafter) Alloy 718. Inconel Alloy 706, A286, and the like may be mentioned. These alloys are excellent in productivity of a relatively large ingot and a gas turbine disk or a rotor shaft material for power generation has been produced from an ingot of about 10 tons.
Furthermore, in the case of the use under a high-pressure hydrogen environment, it is necessary to use a material having low susceptibility to hydrogen embrittlement as a structural material of a pressure vessel. Since strength and ductility are remarkably lowered upon hydrogen embrittlement, the lowering of safety becomes a big problem. In general, a material having higher strength exhibits increased susceptibility to hydrogen embrittlement and, in particular, when a harmful precipitated phase is present, it is known that the susceptibility to hydrogen embrittlement greatly increases. As alloys achieving both of hydrogen embrittlement resistance and high strength, there are, for example, those proposed in PTLs 1 and 2. In PTL 1, it is considered that it becomes possible to attain high strength without increasing the susceptibility to hydrogen embrittlement by subjecting JIS SUH 660 steel (hereinafter. A286 alloy) to cold-working. In PTL 2, it is reported that the susceptibility to hydrogen embrittlement can be reduced by defining the upper limit of the area ratio of NbC in an FeNi-based alloy.